1. Field of the Invention
The present invention relates to a discharge circuit for a pulsed gas laser system such as an excimer or a molecular fluorine laser system. In particular, the present invention relates to a circuit including a solid state switch for providing high voltage, short current pulses to the main laser discharge electrodes of the laser system.
2. Description of the Related Art
Pulsed gas discharge lasers, emitting in the deep ultraviolet (DUV) and vacuum ultraviolet (VUV) region are widely used in various industrial applications such as microlithography, photoablation, and micro-machining, among others. For microlithographic applications, currently used systems include line narrowed excimer lasers, such as ArF (193 nm) lasers and KrF (248 nm) lasers, as well as molecular fluorine (F2) lasers emitting at 157 nm. It is desired that these lasers be efficient and exhibit high energy stability at high repetition rates, for example, at 1-2 KHz or more.
In many scientific, medical and industrial applications for excimer and molecular fluorine gas discharge lasers, it is important that the radiation pulses emitted exhibit stabilized output energy. That gas discharge conditions and characteristics may change, particularly during the operation of the laser, affects establishing constant energy of the emitted radiation. Characteristics and conditions of the gas discharge are dependent upon a number of parameters that, with adequate control, may allow significant improvements toward exact reproducibility.
In conventional pulsed gas discharge excimer lasers, a current pulse is generated by a pulser circuit connected to a power supply, and the current pulse is applied across a pair of main electrodes in a laser chamber filled with a laser active gas to excite the gas. As industrial applications evolve such that higher repetition rates of the laser pulses are desired, the laser discharge circuit has to provide faster current pulses. Moreover, the desired energies of the output laser pulses place constraints on the minimum voltage that is applied to the main discharge electrodes to produce the pulses.
High voltage thyratron switches have been used in the past in excimer laser pulsed power supply discharge circuits. Thyratron switches are capable of withstanding high voltages, for example approximately 50 kVolts, and are very fast (for example, approximately 100 nanoseconds). However, thyratron switches have the drawback that they are imprecise, incurring approximately 10% to 50% fluctuations in voltage, inconsistent with highly energy stabilized laser output pulses. Moreover, thyratrons suffer from having rather short life spans.
In contrast, solid state switches such as IGBTs, GTOs and thyristers are more precise and exhibit longer lifetimes as switching elements for excimer or molecular fluorine gas lasers. However, conventional solid state switches are typically relatively slow (for example, approximately 1 μsecond). At such low speeds, considerable jitter occurs, and voltage fluctuations are again unsatisfactory. Additionally, typical solid state switches are incapable of handling high voltages, for example, approximately 50 kVolts, or even moderate voltages of approximately 14 to 32 kVolts. Thus, in order for a discharge circuit using a low voltage solid state switch (for example, at approximately 4 to 7 kVolts), to supply sufficiently high current pulses to the electrodes of the laser, a step-up transformer may be used after the switch.
However, it is recognized herein that is would be advantageous to have a discharge circuit for an excimer or molecular fluorine laser which uses a solid state switch and that does not require a bulky transformer to produce current pulses of sufficient intensity for the discharge. Moreover, it is desired to have an excimer or a molecular fluorine laser that demonstrates high energy stability.